1. Field of the Invention
The present invention relates to a rolling motion stability control apparatus for a vehicle, particularly relates to an apparatus for restraining a roll increasing tendency of the vehicle to stabilize the rolling motion.
2. Description of the Related Arts
In order to maintain a stability of a vehicle, there is known heretofore an apparatus for controlling a braking force applied to each wheel by means of a braking system, as disclosed in the U.S. Pat. No. 6,086,168, for example. In the United States Patent, described is “For at least one vehicle-movement dynamics characteristic variable which is indicative of the tendency of the vehicle to tilt about the longitudinal axis, an associated tilting-prevention threshold value is prescribed. The respective characteristic-variable instantaneous value is acquired continuously and compared with the tilting-prevention threshold value. As soon as a characteristic-variable instantaneous value exceeds the associated tilting-prevention threshold value, the wheels which are on the outside during cornering are braked in order to prevent the vehicle tilting about the longitudinal axis of the vehicle.” Also, it is described that the instantaneous values of vehicle-movement dynamics characteristic variables include variables such as the lateral acceleration, the change over time of the lateral acceleration, the attitude angle, the attitude-angle velocity, the change in the attitude-angle velocity over time and the slip angle. And, as a first embodiment of the method, it is proposed that the lateral acceleration is the only vehicle-movement dynamics characteristic variable. With respect to another embodiment, it is described that the lateral acceleration, the lateral acceleration, the change in the lateral acceleration over time, the attitude angle, the attitude-angle velocity, the change in the attitude-angle velocity over time and the slip angle are used as vehicle-movement dynamics characteristic variables. And, it is described that if one or more characteristic variables exceed the associated threshold values, the vehicle-movement dynamics control system brakes the wheels which are on the outside during cornering, such that they are placed in a high slip state. In the United States Patent, further described is “If the vehicle-movement-dynamics characteristic variables which are indicative of the tendency of the vehicle to tilt drop back below the associated tilting-prevention threshold values as a result of the vehicle-movement dynamics operating state which then arises, the braking of the wheels which are on the outside during cornering is reduced.”
According to the method as described in the above United States Patent, it focused on a vehicle behavior during cornering, which is a motion about a longitudinal axis of the vehicle, to be classified as a rolling motion in view of a vehicle motion control, as described in a book entitled “Vehicle Dynamics and Control” written by Masato Abe, and published by Sankaido Co. Ltd., on May 31, 1994. On Page 3 of that book, the rolling motion has been classified as a motion which is basically created by a steering operation, together with a lateral motion of the vehicle, and a yawing motion about a vertical axis of the vehicle. And, it is described on Page 148 of the book that if a rolling moment acts on the vehicle, a roll angle will be caused about a roll center.
Referring to FIG. 14, will be explained an example of operation according to the method as described in the above United States Patent. In the case where the lateral acceleration is provided for the vehicle movement characteristic variable indicative of a roll increasing tendency of the vehicle, and (Gyo) is provided for a threshold value, for example, there is shown in FIG. 14 such a state that when the vehicle is moving straight, a steering wheel is rapidly steered by a vehicle driver to be turned to the left (from the time (t00) to the time (t03) in FIG. 14), then counter-steering of the steering wheel is made to be turned to the right (from the time (t03) to the time (t07)), and after being turned to the left (from the time (t07) to the time (t08)), the vehicle returns to the straight movement gradually (from the time (t08) to the time (t10)). In a lower section of FIG. 14, a characteristic for a front right wheel is indicated by (fr), and a characteristic for a front left wheel is indicated by (fl). In this case, at the outset, a clockwise rolling motion with respect to the moving direction of the vehicle is created on the vehicle, then a counter-clockwise rolling motion is created by the counter-steering of the steering wheel, and finally the clockwise rolling motion is created, to return to the straight movement.
As shown in FIG. 14, if the lateral acceleration exceeds the threshold value (Gyo), braking force is applied to a front wheel located at the outside of a curve during cornering operation of the vehicle (at the time (t01) and the time (t06)), whereas if it is equal to or smaller than the threshold value (Gyo), the braking force is reduced (at the time (t04) and the time (t09)). Therefore, the braking force applied to the front wheel located at the outside of the curve varies as shown in the lower section of FIG. 14. However, with the braking force being applied to the front wheel located at the outside of the curve, there might be a case where the rolling motion is affected. For example, if the braking force is applied to the front right wheel, the rolling motion will be created clockwise with respect to the moving direction of the vehicle. In this state, if the braking force is reduced, the rolling motion will be reduced to create the counter-clockwise rolling motion. Particularly, in such a case that the counter-steering is made to change the turning operation from the left to the right, as shown in FIG. 14 from the time (t03) to the time (t07), the braking force applied to the front right wheel is reduced, thereby to cause the counter-clockwise rolling motion, and thereafter the braking force is applied to the front left wheel, thereby to cause the further counter-clockwise rolling motion. This rolling motion resulted from increase and decrease of the braking force applied to the wheels will coincide with the rolling motion resulted from the turning (cornering) operation of the vehicle, in their directions, whereby discontinuity of the rolling motion might be caused.